I have a following scenario I would like some technical help.

Utility boiler with feedwater pump HP discharge side water hammer. There are one inlet and two discharge (HP and IP) for this FW pump.

HP side has no check valve after pump, but has ARC valve for recirc to drum. Whenever unit starts up, we get a water hammer in the HP discharge line. Unit has been operational for 7 years and only started having issues this year. IP discharge has no spillback but has check valve, and does not experience water hammer.

I am thinking the ARC valve is leaking, causing depressurization and flashing in the line while the unit is bottled up at night (we also shut down the feed pump at night and start it up in the morning). So when we start the feed pump, the steam in the water line causes a water hammer. Does this sound possible?

Also, should I consider adding a check valve after the pump? I am not the original engineer for this system. For all pump design, I had always designed with check valve @ discharge to protect the pump. However, I am not familiar with ARC valve. Do they perform the same function (i.e. isolate the pressure) when the boiler feed pump is off?

I would think that when the boiler is off the ARC valve is closed, but perhaps not completely.  Arc valves, being a control valve of a kind, should not be relied upon for providing full closure and should be backed up with a block valve, if full closure is required.

You will have to take a look at the thermodynamic state of your discharge line at the time of startup to guess what is actually happening there.  One would think that on shutdown, hot liquid might be left in the discharge line, which after a pressure loss throught the ARC valve might flash and continue to pass vapor and liquid through the valve.  It is also possible that just cooling could result in a vapor pocket of some kind, as the volume of liquid water reduces it would allow space for vapor to form.  Upon further cooling you might be left with both liquid and vapor at a lesser temperature in the discharge line.  If so, recompressing that discharge line upon startup would most certainly tend to collapse that vapor pocket, which could result in a severe waterhammer.  The line should be vented of any vapor before restart.

Hard to say for sure, but certainly plausible.

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